1. Field of the Invention
The present invention relates to an optical communication device, and in particular relates to means to reduce reception error rates in an optical code division multiplexing (OCDM) transmission/reception device.
2. Description of Related Art
In metropolitan areas which are positioned midway between long-distance networks (which may also be called the “Internet backbone”) and access lines, there is a mounting need for communication at faster communication speeds and with larger data capacity. This is because, whereas the bitrate of the existing Internet backbone is secured in the terabit-per-second communication range, in metropolitan areas communication speeds and capacities lag behind. Hereafter, in consideration of such circumstances as the spread of Internet use and increasing broadband content transmission, faster and higher-capacity communication in metropolitan areas is desired.
In order to increase communication capacity, optical multiplexing technology, in which optical pulse signals for a plurality of channels are combined and transmitted in a single optical fiber transmission path, are being studied. As optical multiplexing technology, active research is in progress on optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), and OCDM. Among these, OCDM has the feature of excellent flexibility in practical applications; that is, per-bit limits in the time domain imposed in OTDM and WDM are not imposed in OCDM.
OCDM is a communication method in which different codes (patterns) are allocated to each channel, and pattern matching is used to extract signals. That is, OCDM is an optical multiplexing technique which uses a method in which, on the transmitting side, optical pulse signals are encoded using different codes for each communication channel, and on the receiving side the same codes as on the transmitting side are used to perform decoding to reproduce the original optical pulse signals.
During decoding, processing is performed in which only optical pulse signals for which codes match are extracted as valid signals, so that it is possible to assign optical pulse signals comprising light at the same wavelength, or the same combination of wavelengths, to a plurality of communication channels. Moreover, a super-structure fiber Bragg grating (SSFBG) or other receiving optical element can be used as the optical encoder, so that there are no electrical limits imposed, and faster signal rates can be accommodated. Further, a plurality of channels at the same wavelength can be multiplexed at the same time, enabling transmission of large amounts of data. OCDM is attracting interest for the dramatic improvement in communication capacity afforded in comparison with OTDM and WDM.
Encoding and decoding methods include direct spreading methods, time-spreading methods, wavelength-hopping methods, and time-spreading wavelength-hopping methods. Hereafter, codes used in time-spreading wavelength-hopping methods are called time-spreading wavelength-hopping codes. This invention relates to OCDM using time-spreading wavelength-hopping codes (see for example Japanese Patent Laid-open No. 2000-209186, and “Enhancement of transmission data rates in incoherent FO-CDMA systems”, X. Wang and K. T. Chan, OECC 2000, Technical Digest, July 2000, 14A2-5, pp. 458-459).
In order to explain the features of an optical code division multiplexing transmission/reception method of this invention and of a device which implements such a method, first the processes of encoding and decoding by OCDM using a time-spreading wavelength-hopping code are explained, referring to FIG. 1. FIG. 1 is a summary block diagram of an optical code division multiplexing transmission/reception device (which hereafter may be called an “OCDM transmission/reception device”). FIG. 1 shows only a portion for one channel of the OCDM transmission/reception device, in order to focus on the processes of encoding and decoding.
In FIG. 1 and subsequent drawings, optical fibers and other optical pulse signal paths are indicated by thick lines, and electrical signal paths are indicated by thin lines. Numbers assigned to these thick lines and thin lines may refer to the paths themselves, or may mean the optical pulse signals or electrical signals propagating in the respective paths. Further, when there is a need to distinguish between a path and the signals propagating in the path, in addition to assigning a number to a path to indicate the path, an arrow may be written along the path and a number provided for the arrow, to indicate the signals propagating in the path.
The OCDM device is configured with the transmission unit 10 and reception unit 20 connected by a transmission path 28. The transmission path 28 is an optical fiber. The transmission unit 10 comprises a light source 12, optical pulse train generator 14, optical modulator 16, and encoder 18. The reception unit 20 comprises a decoder 22, photoelectric converter 24, and receiver 26. In the transmission unit 10, encoded optical pulse signals are generated and transmitted to the reception unit 20. In the reception unit 20, encoded optical pulse signals are received, decoded to reproduce the transmitted optical pulse signals, and the information thus sent is recognized.
The light source 12 is a multi-wavelength continuous-wave light source. Here, for convenience of explanation the light source 12 is assumed in the explanation to be a light source which outputs continuous-wave (CW) light comprising wavelength components at the different wavelengths λ1, λ2 and λ3. However, in general the types of wavelengths comprised by optical pulses are not limited to three types, and even when two types, or four or more types, are comprised, the following explanation similarly obtains. In the following explanation, a light source which outputs CW light may be called a CW light source.
The CW light 13 output from the light source 12 is input to the optical pulse train generator 14, and an optical pulse train 15 is generated and output. An electrical clock signal from an electrical clock signal generator 78 is supplied to the optical pulse train generator 14, and by this means an optical pulse train 15 synchronized with the electrical clock signal is generated.
The optical pulse train 15 is input to the optical modulator 16 and is intensity-modulated in a RZ (Return-to-Zero) format to generate optical pulse signals 17, which are output. A transmission signal from the transmission signal generator 88 is supplied to the optical modulator 16, and by this means the optical pulse train 15 is converted into optical pulse signals 17. Hence the optical pulse signals 17 are optical pulse signals which reflect the binary digital electrical signals supplied by the transmission signal generator 88 to the optical modulator 16.
Below, references to optical pulse signals are used only to mean a train of optical pulses reflecting binary digital electrical signals, obtained by intensity modulation of an optical pulse train to convert electrical pulse signals into optical pulse signals. On the other hand, references to an optical pulse train refer to the entirety of optical pulses arranged at regular fixed time intervals on the time axis.
Optical pulse signals 17 are input to the encoder 18 and encoded, and sent over the transmission path 28 as encoded optical pulse signals 19. The encoded optical pulse signals 19 propagate over the transmission path 28 and are sent to the reception unit 20. In the reception unit 20, the encoded optical pulse signals 19 are input to the decoder 22 and decoded, generating decoded optical pulse signals 23 which are output. The same code as the code set in the encoder 18 is set in the decoder 22, and encoded optical pulse signals 19 are input to the decoder 22 to generate decoded optical pulse signals 23 having the same time waveform as the optical pulse signals 17 during transmission, which are output.
The decoded optical pulse signals 23 are input to the photoelectric converter 24 to generate electrical pulse signals 25, which are input to the receiver 26. The same code as the code set in the encoder 18 is set in the decoder 22, and so decoded optical pulse signals 23 output from the decoder 22 are generated as auto-correlated waveforms of the optical pulse signals 17 generated by the optical modulator 16 in the transmission unit 10. That is, in the reception unit 20, signals sent from the transmission unit 10 (transmission signals output by the transmission signal generator 88) are received in the receiver 26.
The signal transmission process in the above-described OCDM transmission/reception device is explained referring to the time waveforms at different times. FIG. 2A, FIG. 2B and FIG. 2C, as well as FIG. 3A, FIG. 3B and FIG. 3C show optical pulse signals, time waveforms of encoded optical pulse signals obtained in the encoding and decoding processes, auto-correlation waveforms, and cross-correlation waveforms. FIG. 2A and FIG. 3A are time waveforms of optical pulse signals 17, with time along the horizontal axis in arbitrary scale and the optical intensity along the vertical axis in arbitrary scale. In FIG. 1, the positions at which the time waveforms shown in FIG. 2A, FIG. 2B and FIG. 2C and in FIG. 3A, FIG. 3B and FIG. 3C are observed (indicated by the hollow arrows), as well as the signals corresponding to these time waveforms, are differentiated by appending A, B and C, in parentheses, to the numbers indicating the signals.
In FIG. 2A and FIG. 3A, the period indicated by Tb is the data period. The data period is the time interval on the time axis occupied by one optical pulse carrying one bit of information in the optical pulse signals. In general, n bits of optical pulse signals occupy duration of n×Tb on the time axis.
The light source 12 is a light source which outputs CW light comprising wavelength components with the different wavelengths λ1, λ2 and λ3, and so an optical pulse train generated by the optical pulse train generator 14 comprises wavelength components with the center wavelengths λ1, λ2 and λ3. Hence an optical pulse carrying one bit of information comprises wavelength components with the center wavelengths λ1, λ2 and λ3. In order to represent this in FIG. 2A and FIG. 3A, rectangles are shown stacked at the same times, with different shadings (oblique-line patterns) to facilitate understanding.
The time waveform of the coded optical pulse signals 19 appear in FIG. 2B and FIG. 3B. A specific code (taken to be code 1) is assumed to be set in the encoder 18 in the following explanation. The optical pulse signals 17 are input to the encoder 18 and are encoded with time-spreading and wavelength-hopping using code 1, and are output as encoded optical pulse signals 19. Optical pulses comprised by the optical pulse signals 17 are spectrally analyzed into optical pulses with central wavelengths of λ1, λ2, λ3 and similar, which are arranged as optical pulses comprising single-wavelength optical components on the time axis. The optical pulses comprising single-wavelength optical components may be called chip pulses. In FIG. 2B and FIG. 3B, chip pulses are indicated by rectangles denoted by λ1, λ2 and λ3 as the encoded optical pulse signals 19.
The time interval required for time-spreading wavelength-hopping encoding of one optical pulse (one bit) comprised by the optical signals 17 to effect time-spreading arrangement on the time axis as a chip pulse train is called the code period, and in FIG. 2B and FIG. 3B is indicated by Tc. In FIG. 2A and FIG. 3A, and in FIG. 2B and FIG. 3B, the data period Tb and code period Tc are shown as equal; but in general, even when the data period Tb and code period Tc are not equal, OCDM transmission and reception methods can be implemented (see for example “Enhancement of transmission data rates in incoherent FO-CDMA systems”, X. Wang and K. T. Chan, OECC 2000, 14A2-5, pp. 458 through 459 (2000)).
The time waveforms shown in FIG. 2A and FIG. 3A and in FIG. 2B and FIG. 3B schematically show optical pulses and chip pulses; but in actuality, the waveforms are Gaussian or other bell curve-type intensity waveforms. When representing lengths on the time axis such as the data period Tb or code period Tc, the center of one edge of the rectangle on the time axis is represented as the position of the maximum of the optical pulse or chip pulse time waveform. Hence the interval (data period Tb) on the time axis of adjacent optical pulses, shown as λ1, λ2 and λ3 rectangles stacked at the same time, is represented as the interval between centers on one edge along the time axis of optical pulses, indicated by rectangles. Similar representations are used for chip pulses shown in FIG. 2B and FIG. 3B. The interval between a chip pulse of wavelength λ1 and the next chip pulse of wavelength λ1, with the wavelengths λ2 and λ3 intervening, is represented as an interval between center points of one edge on the time axis of chip pulses represented as rectangles. Hereafter, time waveforms of optical pulses and of chip pulses are similarly represented conceptually as rectangles, and the center-point positions (maximum positions) are represented as center points of one edge on the time axis.
FIG. 2C and FIG. 3C show time waveforms of decoded optical pulse signals 23 output from the decoder 22, showing cases in which the code set in the decoder 22 is code 1 and in which the code is code 2, different from code 1, respectively. By setting the decoder 22 to the code given by code 1, the encoded optical pulse signals 19, spread out in a chip pulse train on the time axis, are again reverse-spread on the time axis to obtain decoded optical pulse signals 23 with the same time waveform as the time waveform of the initial optical pulse signals 17, as shown in FIG. 2C.
Decoded optical pulse signals 23 with the same time waveform as the time waveform of the initial optical pulse signals 17, obtained by setting the same code in the encoder 18 and in the decoder 22, are called auto-correlated waveforms. On the other hand, when different codes are set in the encoder 18 and in the decoder 22, then as shown in FIG. 3C, decoded optical pulse signals 23 with the same time waveform as the time waveform of the optical pulse signals 17 cannot be obtained from the encoded optical pulse signals 19. Decoded optical pulse signals 23 with a time waveform different from the time waveform of the initial optical pulse signals 17, obtained by setting different codes in the encoder 18 and in the decoder 22, are called cross-correlated waveforms.
When SSFBGs are used to configure the encoder 18 and decoder 22, by arranging the grating of the SSFBG positioned in the encoder 18 and the grating of the SSFBG positioned in the decoder 22 such that the incident ends are arranged in a mirror-image arrangement, auto-correlated waveforms are obtained. That is, in order to form the encoder 18 and decoder 22 such that auto-correlated waveforms are obtained, the SSFBGs positioned in the encoder 18 and the decoder 22 should have grating arrangements with the same structure, and the incidence ends of the SSFBGs should be on mutually opposite sides.
Next, the configuration and operation of a conventional device to realize the above-described OCDM transmission and reception are explained, referring to FIG. 4 and FIG. 5. FIG. 4 and FIG. 5 are summary block diagrams of the transmission unit 30 and reception unit 90 respectively of a conventional OCDM transmission/reception device. In FIG. 4 and FIG. 5, a case is shown in which the number of multiplexed channels is two in order to simplify the explanation; but it is clear that the following explanation is not limited to the case of two channels, but applies similarly to multiplexing of three or more channels as well.
As shown in FIG. 4, the transmission unit 30 comprises an optical pulse train generation unit 34, first channel 36, and second channel 38.
The optical pulse train generation unit 34 comprises a multi-wavelength continuous-wave light source 32, electrical clock signal generator 134, and optical pulse train generator 50. The multi-wavelength light source 32 comprises CW light sources 40, 42, 44, 46 at wavelengths of λ1, λ2, λ3 and λ4 respectively, and an optical coupler 48. The CW light output from the CW light sources 40, 42, 44 and 46 is combined by the optical coupler 48 to generate multi-wavelength CW light 49, which is input to the optical pulse train generator 50. An electrical clock signal is supplied from the electrical clock signal generator 134 to the optical pulse train generator 50, and in the optical pulse train generator 50 the multi-wavelength CW light 49 is converted into a multi-wavelength pulse train 51 synchronized with this electrical clock signal. That is, a multi-wavelength optical pulse train 51 is generated and output by the optical pulse train generation unit 34.
The multi-wavelength optical pulse train 51 is input to the optical splitter 52 and divided into a first optical pulse train 53-1 and a second optical pulse train 53-2, which are supplied to the first channel 36 and the second channel 38 respectively.
The first optical pulse train 53-1 is input to the optical modulator 54, converted into optical pulse signals 55 and output, for input to the encoder 56. Upon input to the encoder 56, the optical pulse signals 55 are converted into encoded optical pulse signals 61-1 by the encoder 56, and are input to the multiplexer 62. Binary digital electrical signals which are the transmission signals for the first channel are supplied to the optical modulator 54 from the transmission signal generator 84, and these transmission signals of the first channel are reflected in the optical pulse signals 55.
On the other hand, the second optical pulse train 53-2 is input to the optical modulator 58 and converted into optical pulse signals 59 which are output for input to the encoder 60. The optical pulse signals 59 input to the encoder 60 are converted into encoded optical pulse signals 61-2 by the encoder 60, and are then input to the multiplexer 62. Binary digital electrical signals which are the transmission signals for the second channel are supplied to the optical modulator 58 from a transmission signal generator 86, and these transmission signals of the second channel are reflected in the optical pulse signals 59.
The encoded optical pulse signals 61-1 and 61-2 which have been input to the multiplexer 62 are combined to generate optical code division multiplexed signals 63, which are transmitted to the reception unit 90. The reception unit 90 is represented conceptually by the dashed-line square in FIG. 4, and is shown in detail in FIG. 5.
The structure and functions of the reception unit 90 of a conventional OCDM transmission/reception device are explained referring to FIG. 5. FIG. 5 is a summary block diagram of the reception unit 90. The reception unit 90 comprises a demultiplexer 64, first channel 80 and second channel 82. The first channel 80 comprises a decoder 66, time gate unit 68 and photoelectric converter 70; the second channel 82 comprises a decoder 72, time gate unit 74 and photoelectric converter 76.
Optical code division multiplexed signals 63 transmitted to the reception unit 90 are input to the demultiplexer 64 and divided into first optical code division multiplexed signals 65-1 and second optical code division multiplexed signals 65-2, which are supplied to the first channel 80 and second channel 82 respectively.
The first optical code division multiplexed signals 65-1 supplied to the first channel 80 are input to the decoder 66 and decoded, to generate first decoded optical pulse signals 67, which are output. The first decoded optical pulse signals 67 are input to the time gate unit 68 and subjected to time gate processing, to generate second decoded optical pulse signals 69, which are output. The second decoded optical pulse signals 69 are converted into first channel received signals by the photoelectric converter 70.
In the first decoded optical pulse signals 67, auto-correlated waveforms (hereafter sometimes called “signal components”) and cross-correlated waveforms of the first channel optical pulse signals 55 are superposed; the cross-correlated waveforms are a noise component (sometimes called an “interference component”) with respect to the auto-correlated waveforms. Consequently in order to reduce communication errors and similar and enhance reception quality, this noise component must be eliminated. Time gate processing executed in the time gate unit 68 is the means to eliminate the noise component. Time gate processing is processing in which the reception timing is adjusted for each channel such that the noise component and the signal component are not superposed on the time axis after decoding. Specifically, the time gate is put into the transmitting state (on state) during time zones in which the signal component passes through the time gate unit 68, and the time gate is put into the shutoff state (off state) during time zones in which the noise component passes through the time gate unit 68, so that only the signal component can pass through the time gate. After this processing, the signals output from the time gate unit 68 are the second decoded optical pulse signals 69.
On the other hand, the second optical code division multiplexed signals 65-2 supplied to the second channel 82 are similarly input to the decoder 72 and decoded, to generate first decoded optical pulse signals 73 which are output. The first decoded optical pulse signals 73 are input to the time gate unit 74 and subjected to time gate processing to generate second decoded optical pulse signals 75, which are output. The second decoded optical pulse signals 75 are converted into second channel received signals by the photoelectric converter 76.
As explained above, the first and second channel transmission signals transmitted from the transmission unit 30 are reproduced in the reception unit 90 as first and second channel received signals. That is, signals can be transmitted from the transmission unit 30 to the reception unit 90 by an optical code division multiplexing method.
The relation between the above-described auto-correlated waveforms and cross-correlated waveforms and time gate processing is explained more specifically, referring to FIG. 4, FIG. 5, and FIG. 6A1 to FIG. 6E2. In FIG. 4 and FIG. 5, numbers in parentheses indicating the signal observed as time waveforms in FIG. 6 are provided in order to distinguish between A1, A2, B1, B2, C, D1, D2, E1, and E2.
First, time-spreading wavelength-hopping codes are explained. In the following explanation, time-spreading wavelength hopping codes may be simply called “codes”, to the extent that confusion does not arise. As an example, a 16-bit code (λ1, 0, 0, 0, 0, λ2, 0, 0, 0, 0, λ3, 0, 0, 0, 0, λ4) is used in the explanation. Here, the number of symbols in a sequence comprising “0”, “λ1”, “λ2”, “λ3” and “λ4” which form codes may be called the code length. In this example, the code length is 16. The numerical sequences which provide codes are called code sequences, and each of the symbols in a code sequence, “0”, “λ1”, “λ2”, “λ3” and “λ4”, may be called a chip. Also, 0 and λ1, λ2, λ3 and λ4 may themselves be called code values.
The SSFBG comprised by the optical encoder is configured by arranging a plurality of unit FBGs; each unit FBG has a different Bragg reflection wavelength. Each of the chips of the code corresponds to each of the unit FBGs as a function of the distance from one input/output end of the SSFBG to the position at which each unit FBG is positioned, and the order of arrangement of each of the chips “0”, “λ1”, “λ2”, “λ3” and “λ4”. That is, the intended position at which each unit FBG is positioned is determined by fixed intervals from an input/output end of the SSFBG, and by positioning one of the chips “0”, “λ1”, “λ2”, “λ3” and “λ4” at the intended position, each chip of the code sequence is associated with each unit FBG. A chip of “0” which is a code value of 0 means that no unit FBG exists at the corresponding position; and code values of “λ1”, “λ2”, “λ3” and “λ4” which are the chips “λ1”, “λ2”, “λ3” and “λ4” mean that unit FBGs having respective Bragg reflection wavelengths of λ1, λ2, λ3 and λ4 are positioned at the positions corresponding to “λ1”, “λ2”, “λ3” and “λ4”.
In the case of unit FBGs the Bragg reflection wavelengths of which are λ1, λ2, λ3 and λ4, the spectrum of Bragg-reflected light reflected by each of the unit FBGs is not completely monochromatic light at λ1, λ2, λ3 and λ4, respectively (light with a spectral half-maximum width of 0), but rather has a fixed width. For example, in the case of a unit FBG for which the wavelength of Bragg-reflected light is λ1, the spectrum of Bragg-reflected light reflected by the unit FBG comprises components at wavelengths which are slightly smaller than, and slightly greater than, λ1, in addition to the component with wavelength λ1. That is, in the case of a unit FBG with Bragg reflection wavelength of λ1, the center wavelength of the spectrum of Bragg-reflected light from the unit FBG is λ1.
Below, the necessity of time gate processing and other matters are explained referring to FIG. 6A1 through FIG. 6E2, assuming that the above-described 16-bit code (λ1, 0, 0, 0, 0, λ2, 0, 0, 0, 0, λ3, 0, 0, 0, 0, λ4) is the code allocated to the first channel as code 1, and another, different 16-bit code (0, 0, λ2, 0, 0, 0, 0, 0, λ4, λ1, 0, 0, 0, 0, λ3) is allocated to the second channel as code 2. That is, code 1=(λ1, 0, 0, 0, 0, λ2, 0, 0, 0, 0, λ3, 0, 0, 0, 0, λ4), and code 2=(0, 0, λ2, 0, 0, 0, 0, 0, λ4, λ1, 0, 0, 0, 0, 0, λ3).
FIG. 6A1 and FIG. 6A2 show time waveforms for one optical pulse comprised by optical pulse signals in the first channel and in the second channel respectively. The optical pulse signals in each channel comprise a plurality of optical pulses; but if encoding, decoding and similar are explained for one optical pulse in an optical pulse signal, the case is similar for the other optical pulses comprised by the optical pulse signal, and so here an example of a single optical pulse is explained. In order to easily distinguish between optical pulses belonging to the first and second channels, shading is applied in drawings to the optical pulse belonging to the second channel and to chip pulses comprised by the optical pulse. The optical pulses shown in FIG. 6A1 and FIG. 6A2 comprise light components at the different wavelengths λ1, λ2, λ3, and λ4. Further, the optical pulse shown in FIG. 6A1 corresponds to an optical pulse comprised by the first optical pulse train 53-1 shown in FIG. 4, and the optical pulse shown in FIG. 6A2 corresponds to an optical pulse comprised by the second optical pulse train 53-2 shown in FIG. 4.
FIG. 6B1 and FIG. 6B2 show time waveforms of the encoded optical pulse signals 61-1 and 61-2 in the first channel and second channel respectively shown in FIG. 4. The code set in the encoder 56 of the first channel is code 1, and so as shown in FIG. 6B1, chip pulses are arranged in the order given by code 1. First a chip pulse of wavelength λ1 is arranged, and the four time slots corresponding to a code sequence of “0”s are blanks at which no chip pulses exist, then, a chip pulse of wavelength λ2 is arranged. Similarly, chip pulses of wavelengths λ3 and λ4 are arranged at the respective corresponding time slots.
Similarly, the code set in the encoder 60 of the second channel is code 2, and so as shown in FIG. 6B2, chip pulses are arranged in the order given by code 2, similarly to the chip pulses for the above-described first channel.
FIG. 6C shows the time waveform for the optical code division multiplexed signal 63. The time waveform shown in FIG. 6C is the result of combination of the time waveforms of the encoded optical pulse signals 61-1 and 61-2 for the first channel and second channel, shown in FIG. 6B1 and FIG. 6B2.
FIG. 6D1 and FIG. 6D2 show time waveforms of first decoded optical pulse signals 67 and first decoded optical pulse signals 73 in the first channel and second channel respectively, shown in FIG. 5. As shown in FIG. 6D1, the decoder 66 for the first channel is set to code 1, so that the optical pulse of the first channel is reproduced as an auto-correlated waveform. In FIG. 6D1, this auto-correlated waveform is shown as rectangles indicated by the wavelengths λ1, λ2, λ3 and λ4, stacked at the same time. Similarly, the decoder 72 for the second channel is set to code 2, and so optical pulses for the second channel are reproduced as auto-correlated waveforms. In FIG. 6D2, this auto-correlated waveform is shown as shaded rectangles indicated by the wavelengths λ1, λ2, λ3 and λ4, stacked at the same time.
In FIG. 6D1 and FIG. 6D2, the chip pulse components other than the auto-correlated waveforms are noise components. Hence gate signals are sent to the respective time gate units 68 and 74 so that they are in the off state during time zones in which these noise components exist, and so that during time zones in which the auto-correlated waveforms exist they are in the on state. In FIG. 6D1 and FIG. 6D2, time zones in which the time gate units 68 and 74 are in the on state are surrounded by dashed-line rectangles. That is, in FIG. 6D1 the auto-correlated waveform in the first channel is surrounded by a dashed-line rectangle, indicating that the waveform passes through the time gate unit 68 and is output. In FIG. 6D2, the auto-correlated waveform of the second channel is surrounded by a dashed-line waveform, indicating that the waveform passes through the time gate unit 74 and is output.
FIG. 6E1 and FIG. 6E2 show time waveforms for the second decoded optical pulse signals 69 and second decoded optical pulse signals 75 of the first channel and second channel respectively, shown in FIG. 5. Only the auto-correlated waveforms of the first and second channels are allowed to pass through the time gate units 68 and 74 respectively, and cross-correlated waveform components are eliminated. That is, only one optical pulse among the optical pulses comprised by the optical pulse signals of the first channel and second channel, shown in FIG. 6A1 and FIG. 6A2 respectively, is reproduced as the second decoded optical pulse signal 69 and second decoded optical pulse signal 75.
However, when using an optical code division multiplexing transmission/reception method employing conventional time-spreading wavelength-hopping codes comprising the above-described time gate processing means, when the degree of multiplexing (the number of channels multiplexed) is high, and when the code period Tc is set to be short, cross-correlated waveform components appear at positions close to the auto-correlated waveforms on the time axis, so that there are cases in which the cross-correlated waveform components cannot be completely eliminated even when time gate processing means is executed.
In optical code division multiplexing transmission/reception methods using time-spreading wavelength-hopping codes, the signal-to-noise ratio (SNR), defined as the ratio of the intensity of auto-correlated waveforms obtained by decoding to the cross-correlated waveform intensity (maximum peak value), is defined by equation (1) below.
                                                        SNR              =                              SNRn                /                                  (                                      G                    ×                    M                                    )                                                                                                        =                                                (                                      SNRn                    ×                    Tb                                    )                                /                                  (                                      Tc                    ×                    M                                    )                                                                                        (        1        )            
Here SNRn is the value of the SNR for a case in which only a single channel is transmitted by the optical code division multiplexing transmission/reception method, without performing multiplexing, and moreover Tb is set to Tc. M is the degree of multiplexing, that is, the number of channels multiplexed. G is a value defined to be equal to Tc/Tb.
In a high-speed optical communication system in which data is transmitted at high bitrates, a short code period Tc is accommodate by making the data period Tb short. In this case, G is increased, so that the SNR is lower. That is, as the speed of the optical communication system is increased, the SNR is lowered. Further, as the number of channels multiplexed is increased (as M is made larger), the SNR is again lowered.
The following is a more specific explanation of the cause of the above-described lowering of the SNR. As an example, a case is assumed in the explanation in which, for four-channel multiplexing (M=4), the code period Tc is four times the data period Tb (G=4), and the codes allocated to the first through fourth channels are given by the following code1 through code4.                code1=(λ1, 0, 0, 0, 0, λ2, 0, 0, 0, 0, λ3, 0, 0, 0, 0, λ4)        code2=(0, 0, λ2, 0, 0, 0, 0, 0, λ4, λ1, 0, 0, 0, 0, 0, λ3)        code3=(0, 0, 0, 0, λ3, 0, λ1, 0, 0, 0, 0, 0, 0, λ4, 0, λ2)        code4=(0, 0, 0, 0, 0, 0, λ4, 0, 0, λ3, 0, 0, λ2, 0, 0, λ1)        
Given the above assumptions, FIG. 7A through FIG. 7D show, within the range of the data period Tb, time waveforms of decoded optical pulse signals obtained by receiving and decoding optical code division multiplexed signals. In FIG. 7A through FIG. 7D, the horizontal axis is the time axis, and indicates the range of a data period Tb. The vertical axis indicates the optical intensity, on an arbitrary scale.
The time waveforms for the decoded optical pulse signals shown in FIG. 7A through FIG. 7D assume a case in which G=4 and also M=4, so that the respective chip pulse trains comprising 16 chip pulses obtained by decoding the respective optical pulses existing adjacently are arranged on the time axis as follows. That is, in the time zone in which the chip pulse train obtained by decoding one optical pulse corresponding to one bit is arranged, 12 chip pulses of a chip pulse train generated by decoding the optical pulse corresponding to the next adjacent bit exist overlapping on the time axis, in the amount of the time zone.
FIG. 7A, FIG. 7B, FIG. 7C and FIG. 7D show time waveforms of decoded optical pulse signals for the first, second, third, and fourth channels respectively. In FIG. 7A, FIG. 7B, FIG. 7C and FIG. 7D, the symbols I, II, III and IV indicate auto-correlated waveform components and cross-correlated waveform components comprising chip pulses from the first, second, third, and fourth channels respectively. Chip pulses which are unshaded are chip pulses comprising auto-correlated waveform components; shaded chip pulses indicate chip pulses comprising cross-correlated waveform components.
When the code set in the decoder comprised by each channel matches the code set in the encoder of the respective channel, chip pulses from the respective channel overlap in the same time slot within the same data period. That is, when an optical pulse exists in a time slot comprised by a transmitted optical pulse signal, the optical pulse is reproduced, and when no pulse exists, an optical pulse is not reproduced.
In the time waveform of a decoded optical pulse signal of the first channel, shown in FIG. 7A, chip pulses from the first channel (indicated by an unshaded rectangle) are stacked in the leading time slot position in the data period, and one optical pulse is reproduced. Similarly in the time waveforms of decoded optical pulse signals of the second, third and fourth channels in FIG. 7B, FIG. 7C and FIG. 7D respectively, chip pulses from the second, third and fourth channels (indicated by unshaded rectangles) are stacked in the leading time slot positions in the data period (the same time slot position in the same data period), reproducing one optical pulse.
In FIG. 7A, FIG. 7B, FIG. 7C and FIG. 7D, shaded rectangles represent chip pulses from channels other than the respective channels, that is, noise components. In the time waveform of the decoded optical pulse signal for the first channel shown in FIG. 7A, the chip pulses from channels other than the first channel, indicated by II, III and IV, are represented by shaded rectangles. Similarly in the time waveform of the decoded optical pulse signal of the second channel shown in FIG. 7B, chip pulses from channels other than the second channel, indicated by I, III and IV, are represented by shaded rectangles. Time waveforms of decoded optical pulse signals for the third and fourth channels, shown in FIG. 7C and FIG. 7D, are also similarly represented.
Different wavelength components in the time zone forming the same data period at the time of transmission are not superposed at the same position on the time axis after decoding. However, chip pulses arising from the time zone forming different data periods may be superposed at the same position on the time axis after decoding. Hence there are a total of five states of chip pulses forming cross-correlated waveforms, which are noise components, superposed at the same position on the time axis: no chip pulses exist, one chip pulse exists, two superposed chip pulses exist, three superposed chip pulses exist, and four superposed chip pulses exist.
Chip pulses forming cross-correlated waveforms which are noise components occur randomly in five superposed states on the time axis. When optical pulse signals are decoded using a time-spreading wavelength-hopping code, the higher the density of optical pulses comprised by an optical pulse signal, the higher is the frequency of superpositioning of at the same position on the time axis after decoding of chip pulses occurring in time zones forming different data periods. As an extreme case, when optical pulse signals in each of the channels completely fill the time slots in which all optical pulses are placed, four chip pulses forming cross-correlated waveforms which are noise components exist superpositioned in one position on the time axis.
Specifically, for binary digital signals in which “1” indicates an optical pulse exists and “0” indicates no optical pulse, the above corresponds to a case in which all bits are “1”, as in “1, 1, 1, 1, 1, 1, 1, . . . ”.
Chip pulses are not optical pulses which have exactly a single wavelength; rather, the optical spectrum has a finite width. Consequently the time gate signal comprises not only the auto-correlated waveform component, but noise components as well. That is, in conventional methods the edges on the frequency axis of chip pulses forming cross-correlated waveforms are superposed on each other, and as a result cause beat noise to occur.
Hence an object of this invention is to provide an OCDM transmission/reception method and OCDM transmission/reception device using which decoded optical pulse signals free of the above-described noise can be obtained, even when numerous channels are multiplexed, or when the code period is set to be short. As a result, optical communication with a low reception error rate can be realized.